Simulated-fuel-jet/shock-wave interaction

Simulated-fuel-jet/shock-wave interaction

Frank Houwing1, Alexis Bishop2, Matthew Gaston3, Jodie Fox1, PaulDanehy4, and Neil Mudford5

1 Australian National University, ACT 0200, Australia2 The University of Queensland, QLD 4072, Australia3 University of Technology Sydney, NSW 2007, Australia4 NASA Langley Research Center, Hampton, VA 23681-2199, USA5 University of New South Wales, ADFA, ACT, 2600, Australia

Abstract. This paper considers the breakdown of streamwise vorticity as a meansof enhancing the rate of fuel-air mixing in supersonic combustion. In particular, theprospect is investigated of improving fuel-air mixing in a scramjet engine through baro-clinic torque and the amplification of streamwise vorticity arising out of the interactionbetween an oblique shock and a fuel jet issuing from a hypermixing fuel injector.

1 Introduction

This paper considers the breakdown of streamwise vorticity as a means of en-hancing the rate of fuel-air mixing in a supersonic combustion ramjet (scramjet)engine[5]. In particular, the prospect is investigated of improving fuel-air mix-ing in a scramjet engine through the application of baroclinic torque and theamplification of streamwise vorticity associated with a fuel jet produced by ahypermixing fuel injector[7]. This is accomplished by studying the interactionof the flow from a hypermixing fuel injector with an oblique shock wave, usingplanar laser-induced fluorescence (PLIF) imaging of nitric oxide (NO) presentin trace amounts in the simulated fuel. In order to study the flow free of thecomplicating effects of combustion, helium replaces hydrogen in the simulatedfuel stream of the model scramjet combustor. Nitrogen replaces air in the sim-ulated oxidant stream to eliminate NO there. LIF is then produced in mixturesof simulated fuel and oxidant but, ideally, not in the pure oxidant stream.

2 Theoretical Considerations on Shock-VortexInteractions

Figure 1 (a) is a schematic that represents a single fuel jet, with no accompanyingstreamwise vorticity, interacting with an oblique shock wave. Because the fuel jetcontains fluid at a density lower than the surrounding coflow, the interaction withthe oblique shock wave causes the jet to acquire streamwise vorticity throughthe phenomenon of baroclinic torque[8]. This is produced by the interaction oforthogonal components of the gradients of density and pressure. Specifically, thebaroclinic source term in the vorticity transport equation is given by[4]

ρD

Dt

(−→ωρ

)=

1ρ2 ∇ρ × ∇p . (1)

2 Frank Houwing et al.

When a fuel jet is accompanied by streamwise vorticity, an important issue is howthe shock wave and vorticity influence each other. The shock-vortex interactiongenerally leads to three-dimensional curvature of the shock front because of thestrong gradients of total pressure and Mach number in a supersonic streamwisevortex[6].

Theoretical inviscid analysis[3] has shown that the magnitude of the vorticitycomponent tangential to the shock front is increased during interaction with ashock wave, while the magnitude of the normal component is unchanged. Thisanalysis shows that the vorticity jump δ−→ω across a general three-dimensionalsteady curved shock wave is given by

δ−→ω n = 0 , (2)

δ−→ω t = −→n ×[

∇t (ρ1V1n)δρ

−−→V 1t · ∇t

−→V 1tδρ

ρ1V1n

], (3)

where: −→ω = −→ω t + −→ω n is the vorticity vector; −→V is the velocity vector; ρ is the

fluid density; −→n is the unit vector normal to the shock surface; ∇t is the surfacegradient operator; subscripts t and n denote components tangential and normalto the shock surface respectively; δρ ≡ ρ2 − ρ1 and δ−→ω ≡ −→ω 2 − −→ω 1, wheresubscripts 1 and 2 denote conditions immediately upstream and immediatelydownstream of the shock front, respectively.

A fuel jet produced by a hypermixing fuel injector is generally accompaniedby multiple streamwise vortices created by the production of streamwise vorticityin the external flow over specially designed structural features such as compres-sion and expansion ramps, vanes etc. Flow separation at the injector trailing edgeplaces these vortices in close proximity to the fuel jet whereupon they proceed to

38 60 15

131

131

80

25

25

plan view

wedge

fuelstrut

fuelinjector

(c)

16mm8mm

10o

7.5o

16mm

24mm

8mm

(d)

Fuel jet

Oblique shock wave Distorted jet

(a)

22

112

75

14

side elevation

wedgefuel strut

25o

(b)

Fig. 1. (a) Fuel jet interacting with oblique shock wave. (b, c) Fuel injection strut andshock wave generator - side elevation and plan view. (d) Swept compression expansionramp (SCER) hypermixing fuel injector. Dimensions in millimetres.

Simulated-fuel-jet/shock-wave interaction 3

influence the development of the mixing layer. The jet-vortex-shock interactionfor these jets is therefore highly complex, which is why experiment is crucial tothe study of the behaviour of the interaction.

One such injector, known as the swept compression expansion ramp (SCER)injector[2], produces a pair of counter-rotating streamwise vortices for each fueljet. The presence of these streamwise vortices close to the jet/coflow interfacepromotes mixing in the near wake by spreading out the jet thereby increasingthe interfacial area between the two fluids. In addition, the counter-rotation ofthe two vortices tends to split each jet in two[1] even without the presence ofan intersecting shock. This previous work provided motivation for the currentstudy, which seeks to determine whether interaction of the simulated-fuel jetswith an oblique shock wave can increase both the streamwise vorticity and jetbifurcation and thereby promote mixing beyond what can be achieved with thehypermixing injector alone.

3 Experiment

3.1 Flow Conditions and Model

The experiments were performed using the Australian National University’s free-piston shock tunnel T3, which generates a supersonic nitrogen flow to simulatethe coflowing oxidant stream around the model fuel strut. A conical nozzle witha 125 mm diameter exit and 35 mm diameter throat produces a free stream flowwith a static pressure of 40 kPa, a static temperature of 700 K and Mach numberof 4.8.

Figures 1 (b and c) illustrate the experimental apparatus which producesthe desired interactions in the tunnel test section. As this figure shows, a fuelinjector is mounted in the aft section of a horizontal sharp-nosed flat plate whichsimulates a fuel strut in a scramjet engine. The strut is at zero incidence andzero yaw with respect to the oncoming flow. A second sharp-nosed flat plate isset at incidence below the fuel strut. The leading edge of this second plate isslightly forward of the fuel strut trailing edge and parallel to it. The attachedoblique shock off this lower plate then intersects the helium jets issuing from thefuel strut.

The supersonic free stream flow passes over the fuel strut. The simulated fuelconsists of a mixture of 1% CO2, 1.5% NO and 97.5% He injected from the fuelinjector at a Mach number of 1.64, a pressure of 40 kPa and a temperature of160 K. CO2 is an efficient quencher that reduces the fluorescence lifetime of NO.It is added to the mix to provide sufficient quenching to avoid motional blurring,which is the loss of spatial resolution caused by flow motion during fluorescenceemission.

The emphasis of the current work is on the imaging of these simulated-fueljets rather than of the whole flowfield. Therefore the simulated fuel is dopedwith the PLIF target species (NO) while the simulated coflowing oxidant streamis nominally free of NO.


Figure 1 (d) shows the SCER hypermixing fuel injector used here. It is basedon the design used in previous investigations by Gaston et al[2] and Fox etal[1]. It contains six fuel-jet ports, with each port located in the base of a sweptcompression ramp and flanked by expansion ramps. The ramp height is 8.0 mm,the port diameter is 4.0 mm and the port centreline is midway up the ramp.

3.2 Fluorescence Imaging System

The two optical arrangements (A and B) of the laser and camera are illustratedin Fig. 2 (a and b). The laser sheet illuminates one vertical streamwise sectionand four horizontal streamwise sections of the flow, one section at a time. Ineach case, the line of sight of the CCD camera, which captures and focuses thefluorescence from the illuminated planes, is perpendicular to the laser sheet.

The five images are denoted as follows. Vert is the image of the verticalsection which intersects the SCER central injector ports as shown in Fig. 2 (c).Horiz1 is the image of the horizontal section coincident with the strut horizontalplane of symmetry, as shown in Fig. 2 (d). Horiz2 is the image of the horizontalsection which passes through the centres of the three SCER upper injector ports.The horizontal section imaged in Horiz3 lies just above the tops of the SCERupper ports and that imaged in Horiz4 lies just above the tops of the SCERupper compression ramps.

Window

Flow

Injectorstrut

JetLaser sheet

Intensified CCD Camera

Shock tunnel nozzle

Shock tunnel nozzle

Window

Flow

Injector strut

WindowJet

Laser sheet

Intensified CCD Camera

Imaging Configration A

Imaging Configration B

Window

h

FLOW

illuminated region(100 mm wide)

imaged region:576 x 384 pixels78.1 x 52.1 mm

h

FLOW

illuminated region(100 mm wide)

imaged region:576 x 384 pixels78.1 x 52.1 mm

(a)

(b)

(c)

(d)

Fig. 2. Optical configurations: (a) For capture of the vertical section images, (b) Forcapture of horizontal section images. (c) Illuminated and imaged regions of verticalsection, (d) Illuminated and imaged regions of horizontal sections; case shown is sym-metry plane section. The sharp nosed flat plate at incidence and its accompanyingoblique shock are omitted for clarity.


4 Results and Discussion

Figure 3 shows a set of the images produced by PLIF, as described above. Theseimages all employ the same logarithmic greyscale in which light and dark indicatehigh and low fluorescence signal intensity respectively.

It is apparent from the figure that some features, such as shock waves, arevisible far from the jet even though the flow there should be free of NO. A spectralfilter in front of the ICCD camera severely attenuates the scattered laser light sothat Rayleigh scattered light, which is of low intensity in the tunnel, should beeliminated. A Mie scattering signal should be coarser in appearance. It thereforeseems likely that it is LIF, implying that minute quantities of oxygen may have

(f)

(d)

(b)(a)

(h)

(e)

(g)

(c)

Fig. 3. PLIF images of simulated fuel. Oncoming flow is left to right. (a) Vert plane-base injector, oblique shock. (b) Vert SCER injector, no oblique shock. (c) Horiz2SCER injector, no oblique shock. (d) Vert SCER injector, oblique shock. (e) Horiz1SCER injector, oblique shock. (f) Horiz2 SCER injector, oblique shock. (g) Horiz3SCER injector, oblique shock. (h) Horiz4 SCER injector, oblique shock.


contaminated the test gas leading to NO production in the shock tube. Whateverthe signals’ origin, it is not of concern for our present purpose as the image ofthe seeded jet is clear and laser attenuation due to NO in the simulated oxidantstream will be negligible. In any case, it is helpful for interpretation to see theseotherwise invisible features.

Figure 3 (a) shows Vert of a single helium jet interacting with the obliqueshock. This jet issues from a plane base injector installed in place of the SCERinjector. Apart from this change, the optical and flow arrangements are as shownin Fig. 2 (c). Figure 3 (a) shows the jet to be largely unaffected by the shock,apart from turning with the flow as it enters the shock. In particular, littleor no jet bifurcation is observed. The absence of such bifurcation is supportedby horizontal plane images not presented here. This jet and mixing layer areaccompanied by very little streamwise vorticity as a plane base injector has nocompression or expansion ramps to generate it. It is therefore an example of theinteraction illustrated in Figure 1 (a).

Figures 3 (b and c) show images Vert and Horiz2, respectively, of jetsissuing from the SCER with no oblique shock present. Fig. 3 (d) shows the Vertimage of the SCER central jets interacting with the oblique shock wave while theremaining images are horizontal views of the same flow realised in other shots. Anumber of observations can be made from comparison of these images. Firstly,the interacting jets have spread out in the vertical plane and turned ahead ofthe plane of the incident oblique shock. This suggests that the otherwise straightoblique shock has been distorted into a curved shock. This curvature is not visiblein the images but this is not surprising as the PLIF signal is relatively insensitiveto pressure and temperature for the chosen laser transitions and conditions. Thecurved shock is expected to be similar to that of a bow wave on a blunt body.Downstream of the shock, the jets bifurcate and consequently disappear rapidlyfrom the illuminated vertical plane.

Figure 3 (e) shows the Horiz1 image of the SCER horizontal symmetryplane. Upstream of the interaction region, only small amounts of jet fluid aretransported into this plane by turbulent mixing in the mixing layer. Hence lowPLIF signal levels are observed here. Downstream of the interaction region, thehelium jets originating from the lower ports are bifurcated by the oblique shockwave and deflected upwards. They consequently intersect the illuminated planeand produce the stronger signals seen on the right of the figure.

The Horiz2 image in Figure 3 (f) clearly shows the initial stages of the jetbifurcation before the jet material moves upwards and out of the illuminatedplane. Before bifurcating, the jets spread rapidly taking up shapes reminiscentof that predicted for flow in the vicinity of a bursting vortex[5].

The Horiz3 and Horiz4 images in Figure 3 (g and h) show the bifurcatedjets further downstream of the interaction region as their deflection by the shockcarries them upwards.

Clearly, the SCER jets show much stronger spreading and bifurcation thanthe plane base injector jet when acted upon by the oblique shock. This impliesthat, in the flows examined here, streamwise vorticity amplification and inter-


action with the shock is a more powerful mixing enhancement mechanism thanbaroclinic torque.

5 Summary and Conclusions

The interaction of a shock wave with helium jets issuing from a hypermixing fuelinjector has been visualised using fluorescence imaging. The observed bifurcationof the jets is attributed to the effects of baroclinic torque and the shock-inducedamplification of streamwise vorticity, particularly the latter. The jet displaysbehaviour consistent with the presence of a bubble shock standing out from theincident plane oblique shock. It has yet to be determined whether this featureis produced by vortex bursting or by variations in total pressure in the mixingflowfield. Further work is required to ascertain whether or not the streamwisevortices have burst.

6 Acknowledgment

This work was supported by a Small ARC Grant No. F00134 funded throughthe Australian National University. The technical expertise provided by Mr PaulWalsh and Mr Paul Tant is gratefully acknowledged.

References

1. J. S. Fox, A. F. P. Houwing, P. M. Danehy, M. J. Gaston, N. R. Mudford, S. L. Gai:Mole-Fraction-Sensitive Imaging of Hypermixing Shear Layers, J. Prop. Power17(2) 284-292 (2001)

2. M. J. Gaston, N. R. Mudford, A. F. P. Houwing: A Comparison of Two HypermixingFuel Injectors in a Supersonic Combustor, AIAA Aerospace Sciences Meeting, Reno,NV, AIAA Paper No. 98-0964 (1998)

3. W. D. Hayes:, The vorticity jump across a gasdynamic discontinuity, J. Fluid Mech.,2, 595–600 (1957)

4. F. E. Marble, E. E. Zukoski, J. W. Jacobs, G. J. Hendricks, I. A. Waitz:,Shock Enhancement and Control of Hypersonic Mixing and Combustion,AIAA/ASME/SAE/ASEE 26th Joint Propulsion Conference, Orlando, FL, AIAAPaper, 90-1981 (1990)

5. A. Nedungadi, M. J. Lewis:, Computational study of the flowfields associated withoblique shock/vortex interactions, AIAA Journal, 34 (12), 2545–53 (1996)

6. M. K. Smart and I. M. Kalkhoran, Effect of shock strength on oblique shock-wave/vortex interaction, AIAA Journal, 33 (11), 2137–43 (1995)

7. I. A. Waitz, F. E. Marble, E. E. Zukoski:, Vorticity generation by contoured wall in-jectors, AIAA/SAE/ASME/ASEE 28th Joint Propulsion Conference and Exhibit,Nashville, TN, USA, AIAA Paper, 92-3550 (1992)

8. J. Yang, T. Kubota, E. E. Zukoski:, An Analytical and Computational Investigationof Shock-induced Vortical Flows, 30th Aerospace Sciences Meeting and Exhibit,Reno, NV, AIAA Paper, 92-0316 (1992)

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Simulated-fuel-jet/shock-wave interaction